A facile three-step synthesis of (±)-crispine A via an acyliminium ion cyclisation

A high yielding cyclisation of the readily available N-(4,4-diethoxybutyl)-2-(3,4-dimethoxyphenyl)acetamide to 8,9-bis(methyloxy)-2,3,6,10b-tetrahydropyrrolo[2,1-a]isoquinolin-5(1H)-one is described. The latter can be reduced with either AlH₃ or BH₃ to (±)-crispine A in an overall yield of 55%.     

Highly chemoselective reduction using a Rh/C-Fe(OAc)2 system: practical synthesis of functionalized indoles

Here, we report a highly effective and chemoselective method of preparing substituted indoles from (E)-2-nitropyrrolidinostyrenes via hydrogenation in the presence of a rhodium catalyst doped by additives such as Ni(NO₃)₂·6H₂O, Fe(OAc)₂ or Co(acac)₃. These hydrogenation conditions may also be applied to other substrates. Aromatic nitro compounds and olefins can be selectively reduced in the presence of aromatic benzyl ethers, aromatic halides and aromatic aldehydes.     

Electrical properties and stoichiometry in La2NiO4

The electrical properties of La₂NiO₄ have been studied with respect to the stoichiometry of the material. The conductivity of reduced compositions has been measured between 20 and 1000 K and compared to that of the air-prepared ones. Nonstoichiometry is present in both cases and produces disorder leading to Anderson localization and to the definition of a mobility edge in the σ_x²−y² itinerant band. Air-prepared compounds contain in addition a large number of Ni³⁺ states which may overlap the itinerant σ_x²−y² band. For reduced materials containing small amounts of Ni³⁺, the electrical properties can be described below 200 K by a hopping conductivity at the Fermi level within a sharply peaked density of states. The results are well described within the frame of the Mott theory of variable range hopping. Above 200 K highly reduced materials exhibit direct excitation of holes from Ni³⁺ states to the mobility edge in the itinerant band. Under conditions appropriate to air-prepared materials, the Fermi level is shifted toward the itinerant band and a major contribution to the conductivity arises from hopping at the Fermi level. At high temperature a progressive excitation of carriers from the localized states is anticipated with an eventual exhaustion region. This last assumption is corroborated with a shift of the conductivity maximum to higher temperature for increasingly reduced materials.     

Oxidation with alkaline permanganate using formic acid for the back-titration : Potentiometric determination of chromium

Determination of chromium by oxidation of chromite with permanganate does not give accurate results. KmnO₄ is reduced to MnO₂. Titration of KmnO₄ with Cr⁺³ solution in the presence of 0.8–1.5N NaOH and Ba⁺² ions yields manganate and gives good results. In the absence of Ba⁺² ions and in the presence of 0.5–2N NaOH reduction of KmnO₄ passes quantitatively to MnO₂.Cr⁺³ can be determined by adding the chromic solution to KmnO₄ while stirring in presence of 1N NaOH and Ba⁺² ions, or a. 2.5N NaOH in the absence ofBa⁺² ions. The excess KmnO₄ is then back-titrated with formic acid.     

NaBH3CN and other systems as substitutes of tin and silicon hydrides in the light or heat-initiated reduction of halosugars: a tunable access to either 2-deoxy sugars or 1,5-anhydro-itols

UV light-promoted reduction of acetobromoglucose by NaBH₃CN in t-BuOH afforded 1,3,4,6-tetra-O-acetyl-2-deoxy-α-d-arabino-hexopyranose in high yield and purity, via a Surzur–Tanner rearrangement, while, with 10 mol % thiophenol added, acetylated 1,5-anhydro-d-glucitol was cleanly obtained. Such tin-free and mild reductions, presumed to proceed via radical pathways, were more efficient with NaBH₃CN compared to NaBH₄ or NaBD₄, and do not occur with acetochloroglucose. Similar reductions to 1,3,4,6-tetra-O-acetyl-2-deoxy-α-d-arabino-hexopyranose were achieved upon heating to 80 °C t-BuOH or CH₃CN solutions of NaBH₃CN and AIBN, but with a lower selectivity due to competing ionic reactions. With other pyranosyl bromides, reductions by NaBH₃CN could be tuned similarly (d-galacto), but some (d-manno, 5-thio-d-xylo) gave mainly or exclusively 1,5-anhydro-itols. Other conditions, or reagents promoting SET process, afforded also reduced products, but with lower rates or selectivities. Primary iodides were reduced readily with NaBH₃CN under UV light.     

Synthesis of octahedral carbonyl complexes of manganese(I) with SCN or CN ligands. Crystal structure of fac-[SCNMn(CO)3(dppm)]

The complexes fac-[XMn(CO)₃(dppm)], cis,cis-[XMn(CO)₂(dppm)(P(OPh)₃)] and trans-[XMn(CO)(dppm)₂] with X = SCN or CN have been prepared from the corresponding bromocarbonyls and the salts AgX or KX, or, in the case of the di- and mono-carbonyls, from fac-[XMn(CO)₃(dppm)] with X = SCN or CN by thermal or photochemical CO substitution by the ligands P(OPh)₃ or dppm. The structure of fac-[SCNMn(CO)₃(dppm)] has been determined by X-ray diffraction. The crystals are monoclinic, space group P2₁/n, and the structure has been refined to R = 0.058 for 4123 reflexions measured in the range 2 ⩽ θ ⩽ 30 at room temperature. The cis,cis-[NCMn(CO)₂(dppm)(P(OPh)₃)] complex can be oxidized and subsequently reduced to the isomer trans-[NCMn(CO)₂(dppm)(P(OPh)₃)]. All the neutral cyanide complexes react readily with MeI and KPF₆ to give the corresponding methylisocyanide derivatives [Mn(CO)₂(dppm)(P(OPh)₃)(CNMe)]PF₆ and [Mn(CO)(dppm)₂(CNMe)]PF₆. The stereochemistries of the compounds is discussed in relation to the ³¹P NMR spectra.     

Synthesis of Hexagonal Co3O4 and Ag/Co3O4 Composite Nanosheets and their Electrocatalytic Performances

Sheet-like precursors of Co₃O₄ and Ag/Co₃O₄ composites with different Ag contents were synthesized with assistance of triethylamine via a hydrothermal process. The final samples were fabricated by calcing each precursor at 400 °C. The as-prepared samples were identified and characterized by thermogravimetric analysis and differential thermal analysis, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and field emission scanning electron microscopy, respectively. The Co₃O₄ and Ag/Co₃O₄ composite samples were used as electrocatalysts modified on a glassy carbon electrode for p-nitrophenol reduction in a basic solution. The electrocatalytic results indicated that all the samples showed enhanced electrocatalytic performance for p-nitrophenol by comparing a bare glass carbon electrode, and p-nitrophenol could be reduced by Co₃O₄ at a high peak current but a rather higher peak potential but be reduced effectively by Ag/Co₃O₄ composites at lower potential. Ag/Co₃O₄ composites with 4 % Ag displayed the highest electrocatalytic activity.     

Non-covalent assembly of proton donors and p-benzoquinone anions for co-electrocatalytic reduction of dioxygen

The two-electron and two-proton p-hydroquinone/p-benzoquinone (H₂Q/BQ) redox couple has mechanistic parallels to the function of ubiquinone in the electron transport chain. This proton-dependent redox behavior has shown applicability in catalytic aerobic oxidation reactions, redox flow batteries, and co-electrocatalytic oxygen reduction. Under nominally aprotic conditions in non-aqueous solvents, BQ can be reduced by up to two electrons in separate electrochemically reversible reactions. With weak acids (AH) at high concentrations, potential inversion can occur due to favorable hydrogen-bonding interactions with the intermediate monoanion [BQ(AH)_m]˙⁻. The solvation shell created by these interactions can mediate a second one-electron reduction coupled to proton transfer at more positive potentials ([BQ(AH)_m]˙⁻ + nAH + e⁻ ⇌ [HQ(AH)_(m+n)−1(A)]²⁻), resulting in an overall two electron reduction at a single potential at intermediate acid concentrations. Here we show that hydrogen-bonded adducts of reduced quinones and the proton donor 2,2,2-trifluoroethanol (TFEOH) can mediate the transfer of electrons to a Mn-based complex during the electrocatalytic reduction of dioxygen (O₂). The Mn electrocatalyst is selective for H₂O₂ with only TFEOH and O₂ present, however, with BQ present under sufficient concentrations of TFEOH, an electrogenerated [H₂Q(AH)₃(A)₂]²⁻ adduct (where AH = TFEOH) alters product selectivity to 96(±0.5)% H₂O in a co-electrocatalytic fashion. These results suggest that hydrogen-bonded quinone anions can function in an analogous co-electrocatalytic manner to H₂Q.

Non-covalent interactions between reduced p-benzoquinone species and weak acids stabilize intermediates which can switch dioxygen reduction selectivity from H₂O₂ to H₂O for a molecular Mn catalyst.     

Electron Capture Dissociation of Hydrogen-Deficient Peptide Radical Cations

Hydrogen-deficient peptide radical cations exhibit fascinating gas phase chemistry, which is governed by radical driven dissociation and, in many cases, by a combination of radical and charge driven fragmentation. Here we examine electron capture dissociation (ECD) of doubly, [M + H]^2+?, and triply, [M + 2H]^3+?, charged hydrogen-deficient species, aiming to investigate the effect of a hydrogen-deficient radical site on the ECD outcome and characterize the dissociation pathways of hydrogen-deficient species in ECD. ECD of [M + H]^2+? and [M + 2H]^3+? precursor ions resulted in efficient electron capture by the hydrogen-deficient species. However, the intensities of c- and z-type product ions were reduced, compared with those observed for the even electron species, indicating suppression of N?CC_?? backbone bond cleavages. We postulate that radical recombination occurs after the initial electron capture event leading to a stable even electron intermediate, which does not trigger N?CC_?? bond dissociations. Although the intensities of c- and z-type product ions were reduced, the number of backbone bond cleavages remained largely unaffected between the ECD spectra of the even electron and hydrogen-deficient species. We hypothesize that a small ion population exist as a biradical, which can trigger N?CC_?? bond cleavages. Alternatively, radical recombination and N?CC_?? bond cleavages can be in competition, with radical recombination being the dominant pathway and N?CC_?? cleavages occurring to a lesser degree. Formation of b- and y-type ions observed for two of the hydrogen-deficient peptides examined is also discussed.     

Effect of nuclear spin of a homonuclear molecule on the rate of ion capture

Calculations employing statistical adiabatic channel model show that the spin of a nucleus in a homonuclear molecule affects the rate of ion capture at low temperatures. Whereas for bosons in even electronic states with respect to permutation of the nuclear coordinates (Σ⁺_g,Σ⁻_u) the ion capture rate decreases as the spin increases, for fermions in such states the situation is reversed; the capture rate increases with increasing spin. For the odd Σ⁺_u,Σ⁻_g states the trends are reversed. Such trends are exhibited over the whole region of low temperatures. At very low temperatures the spin effects can become very large. If the effect of the reduced mass contributed by the Langevin rate constant can be ignored, the normal kinetic order at which the isotopomeric molecules capture an ion is reversed. For instance, C₂T₂+H⁺₃ occurs slower than does C₂D₂+H⁺₃.     

Mixed alkyl isocyanide-1,4-diaza-1,3-butadiene complexes of chromium: A series of 17-, 18- and 19-electron species of the type [Cr(CO)3(CNR)(R′-DAB)]1+,0,1′

The chromium(0) complexes Cr(CO)₃(CNR)(R′-DAB) (R = Me, CHMe₂, CMe₃, or xylyl; R′ = i-Pr, t-Bu, Cy or p-tol) can be prepared by the reaction of Cr(CO)₃(NCMe)₃ with equimolar quantities of R′-DAB and RNC. These complexes are oxidized to the paramagnetic 17-electron salts [Cr(CO)₃(CNR)(R′-DAB)]PF₆ by [(η⁵-C₅H₅)₂Fe]Pf₆ in ethanol, and can be reduced to the paramagnetic 19-electron radical anions [Cr(CO)₃(CNR)(R′-DAB)]⁻ using NaHg in tetrahydrofuran. The spectroscopic (IR, ¹H NMR, electronic absorption and ESR) and electrochemical properties of the [Cr(CO)₃(CNR)(R′-DAB)]^1+,0,1′ species (where appropriate) have been recorded. The dark red monocationic species [Cr(CO)₃(CNR)(R′-DAB)]PF₆ are subject to disproportionation in solution to give separable mixtures of dark blue Cr(CO)₃(CNR)(R′-DAB) and green [Cr(CNR)₄(R′-DAB)](PF₆)₂.     

InCl3 as an efficient catalyst for synthesis of oxazolines under thermal, ultrasonic and microwave irradiations

Efficient synthesis of 2-oxazolines by the reaction of nitriles with β-aminoalcohols using InCl₃ as catalyst under reflux conditions is reported. This catalyst can be successfully applied to the chemoselective conversion of dicyanobenzenes to their corresponding mono- and bis-oxazolines. The application of ultrasonic and microwave irradiation improved the yields and reduced the reaction times. Another advantage of this catalytic system is its ability to carry out large-scale reactions under ultrasonic and MW irradiations. Alkylnitriles such as acetonitrile was also converted to its corresponding 2-methyloxazoline in the presence of catalytic amounts of InCl₃.     

Polarographic studies of catalytic reduction of hydrogen in Cr(VI)−As(III) and Cr(III)—As(III) systems

In the presence of traces of Cr(VI) or Cr(III) ions in ammonia or borate buffers containing the As(III) ions a catalytic hydrogen wave arises in the dc polarogram. It was established that the complex Cr(H₂AsO₃)_n^+3?n is formed in the solution, and that its reduced form adsorbed at DME is of catalytic activity. The wave can be employed for the determination of low concentrations (2×10^?8×10^?7M) of Cr(VI) and Cr(III) ions.     

Reduced dipole pseudospectra for the accurate tabulation of C6 dispersion coefficients

The possibility of tabulating accurate reduced dipole pseudospectra of single molecules is of great importance for obtaining reliable dipole dispersion constants from which C₆ dispersion coefficients can be derived for all possible interacting pairs. Use of an efficient interpolation procedure allows to obtain dispersion constants from analytical integration of the Casimir–Polder integral over the product of frequency-dependent polarizabilities (FDPs) at imaginary frequencies. FDP calculations can then be done at a few selected frequencies, so overcoming at a time the difficulties arising from numerical quadratures and sensibly reducing the computational demand. In the static case, appropriately reduced n-term pseudospectra are obtained by forcing in an optimal way large N-term extended pseudospectra to be converted to FDPs using the same interpolation technique. Calculations performed at the eight frequencies arising by choosing n = 4 (four-term approximation) in the optimized interpolation procedure, give reduced dipole pseudospectra from which isotropic C₆ dispersion coefficients and γ₆ anisotropy coefficients are obtained in almost perfect agreement with the result of the most accurate data available from the literature and of recently proposed variational techniques.     

ÜbergangsmetallCarbin-Komplexe: LXXXVI. Synthese mono-, bis-, und tris-Trimethylphosphan-substituierter Diethylaminocarbin-Komplexe des Wolframs

The reaction of trans-I(CO)₄WCNEt₂ (I) with a slight excess of PMe₃ results in the replacement of one carbonyl group to give mer-I(CO)₃(PMe₃)WCNEt₂ (II). Complex II reacts at room temperature with additional PMe₃ under CO replacement to give a mixture of cis- and trans-dicarbonyl-I(CO)₂(PMe₃)₂WCNEt₂ (III, IV). Complexes III and IV, which can be separated by column chromatography, isomerize slowly at room temperature, the thermodynamic equilibrium favouring the more stable trans complex IV. The cis isomer III can be obtained from I(CO)₂py₂WCNEt₂ (V) and PMe₃. Another CO ligand can be eliminated from III or IV by an excess of PMe₃ in boiling hexane and gives mer-I(CO)(PMe₃)₃WCNEt₂ (VI). Moreover complex VI can be prepared by oxidative decarbonylation from III or IV by iodine and subsequent reduction of the intermediate, an isolable, seven-coordinated carbyne complex formulated as (I)₃(CO)(PMe₃)₂WCNEt₂ (VII), by two equivalents of PMe₃.     

Molecular weight dependence of stress relaxation of commercial polyisobutylene

Stress relaxation measurements in simple extension of three commercial samples of polyisobutylene (PIB) of different viscosity molecular weights (1.1, 1.7 and 2.3 × 10⁶) have been made at stretch ratios (λ) up to about 2. Data obtained at 3° and 50° have been reduced at 25°. As already found previously, instantaneous stress-strain relationship can be expressed by the Mooney-Rivlin equation. The coefficient C₁ and C₂, obtained by two linear plotting procedures show for each sample behaviour already reported viz. a decrease of C₁ with time for short times and a delay of the decrease of C₂ about three decades of time later. C₁ and C₂ both show a marked dependence on molecular weight: the C₂ behaviour is as expected in terms of a tentative molecular interpretation given previously, whereas the C₁ dependence is somewhat unexpected. The latter behaviour could be attributed, however, to the influence of the unknown molecular weight distributions of the samples.     

The electrical,optical and photoconducting properties of Fe2−xCrxO3 (0 ⩽ x ⩽ 0.47)

A study has been made of the electrical, optical and photoconducting properties of pure and reduced single crystals of composition Fe_2?xCr_xO₃ where 0 ? x ? 0.47. It has been found that pure α-Fe₂O₃ is not a photoconductor. When defect-free crystals of α-Fe₂O₃ are reduced a surface layer of Fe₃O₄ is formed and the crystals exhibit photoconductivity. Removal of this layer resulted in the disappearance of photocurrents and an increase in the sample resistivity. A necessary condition for the observation of photocurrents in n-type Fe₂O₃ is that some Fe₃O₄ be present. In addition, it has been found that the substitution of chromium for iron in α-Fe₂O₃ results in a monotonically decreasing optical band gap as the chromium concentration, x, increases.     

Algebraic symmetry and determination of the “imaginary” crystal field parameters from optical spectra of fn-ions. Hexagonal and trigonal symmetry

Algebraic symmetry of the characteristic equation of the crystal field hamiltonian Ĥ_CF for hexagonal (C₆, C_3h, C_6h) and trigonal (C₃, S₆) symmetry is considered. Explicit dependence of the coefficients a_i of the characteristic equation on the CF parameters is derived by ALTRAN. Magnitudes, signs and/or relative signs of the CF parameters which can be determined from fitting the optical spectra of fⁿ-ions at sites of the above symmetry are established. The algebraic results derived here can facilitate the fitting procedures for any hexagonal and trigonal symmetry. The relevant literature is extensively surveyed. The approach C (complete) to fitting the optical spectra, which yields ambiguous and unreliable results in general, has not been employed to hexagonal (C₆, C_3h, C_6h) symmetry. Some misinterpretations on the approach R (reduced) and A (approximated), which merge for this symmetry, are clarified. Crystal field analysis has been carried out for trigonal (C₃, S₆) symmetry in but a few cases only. The survey has revealed one case of the inappropriate C used for Eu³⁺ in the trigonal (C₃) Eu(DBM)₃ · H₂O system.     

Phase stability and non-stoichiometry in M-phase solid solutions in the system LiO0.5-NbO2.5-TiO2

Phase relations at 1050°C have been determined for M-phase solid solutions in the LiO_0.5-NbO_2.5-TiO₂ ternary phase system by the quench method. Rietveld analysis has been used to help determine phase boundaries and to study structure composition relations. The M-phases have trigonal structures based on intergrowth of corundum-like layers, [Ti₂O₃]²⁺, with slabs of (N−1) layers of LiNbO₃-type parallel to (0001). Ideal compositions are defined along the pseudobinary join LiNbO₃-Li₄Ti₅O₁₂ by the homologous series formula Li_NNb_N−4Ti₅O_3N, N?4. Homologues with N?10 lie to the low-lithia side of the LiNbO₃-Li₄Ti₅O₁₂ join and show extended single-phase solid solution ranges separated by two-phase regions. The composition variations along the solid solutions are controlled by a major substitution mechanism, Li⁺+3Nb⁵⁺↔4Ti⁴⁺, coupled with a minor substitution 4Li⁺↔Ti⁴⁺+3□, where □=vacancy. The latter substitution results in increasing deviations from the stoichiometric compositions A_2N+1O_3N with increasing Ti substitution. The non-stoichiometry can be reduced by re-equilibration at lower temperatures. Expressions have been developed to describe the compositional changes along the solid solutions.     

Diastereoselective atom transfer radical cyclization reactions of unsaturated α-bromo oxazolidinone imides catalyzed by Lewis acids

A new Lewis acid-catalyzed atom transfer radical cyclization reaction of unsaturated α-bromo oxazolidinone imides is reported. In the presence of Lewis acids such as Mg(ClO₄)₂ and Yb(OTf)₃, a series of trans cyclic products was obtained in high yield (up to 87%) between 0°C and room temperature. The loading of strong Lewis acids, such as Yb(OTf)₃, can be reduced to 0.1 equiv. without significantly compromising the yield. Excellent diastereoselectivity could be achieved by using 1,2-stereocontrol or a chiral oxazolidinone auxiliary. For substrates 1e and 1f bearing a β-methyl substituent and the chiral auxiliary, (S)-(−)-4-benzyl-5,5-dimethyl-2-oxazolidinone, respectively, the diastereomeric ratio of the products was greater than 50:1.